concurrent-make: preference variable

Default value: 1

The concurrent-make variable controls how the make command
behaves. If 0, the make command always runs the compiler or
other program non-concurrently, exiting the concurrent process if it
needs to. If 2, the make command always runs the compiler
concurrently, creating a concurrent process if it needs to. If 1
(the default), the make command runs the compiler concurrently if
a concurrent process is already running, non-concurrently otherwise.